Implement Authentication Session URL Retrieval in Gateway

[p5150j]

Implement Authentication Session URL Retrieval in Gateway

Summary:

As a developer, I want to implement the functionality in the Gateway to retrieve an AuthenticationSessionUrl, so that the app backend can initiate the sign-in process.

Acceptance Criteria:

• Implement the getAuthenticationSessionUrl() method in the Gateway.
• Generate and sign the request payload (SiwaRequest).
• Ensure that the Gateway communicates with Frequency Access (fa) to post the sign-in request and receives a 201 Created response with a Location header containing the AuthenticationSessionUrl.
• Return the AuthenticationSessionUrl to the app backend.

Tasks:

1. Develop the getAuthenticationSessionUrl() logic in the Gateway.
2. Generate and sign the SiwaRequest payload.
3. Implement the POST request to `/siwa/api/request' endpoint in Frequency Access.
4. Handle the response, extract the AuthenticationSessionUrl from the Location header.
5. Ensure proper error handling and logging.
6. Write unit tests for the method.